Crude oils, and middle distillates obtained by distillation of crude oils, such as gas oil, diesel oil, or heating oil, contain various amounts of dissolved long-chain paraffins (waxes), depending on their origin. At low temperatures, these paraffins precipitate as platelet-shaped crystals, sometimes with the inclusion of oil. This considerably impairs the flow properties of the crude oils and the distillates obtained from them. Deposits of solids occur, which often lead to problems in the production, transportation, and use of the mineral oil products. In the cold season, for example, blockages occur in the filters of diesel engines and furnaces, which prevent reliable metering of the fuels and ultimately can even result in an interruption of the supply of fuel or heating medium. Transportation of the middle distillates through pipelines over relatively large distances can also be impaired in winter by the precipitation of paraffin crystals.
It is known that the undesirable crystal growth can be suppressed by suitable additives, so that the tendency of the viscosity of the oils to increase is minimized. Such additives, which are known pour-point depressants or agents which improve flow, change the size and shape of the wax crystals and, in this way, counteract increases in the viscosity of the oils.
The flow and low-temperature properties of mineral oils and mineral oil distillates are described by the pour point (determined in accordance with DIN 51597) and the cold filter plugging point (CFPP; determined in accordance with DIN 51428). Both parameters are measured in .degree.C.
Typical agents for improving the flow of crude oil and middle distillates are copolymers of ethylene with carboxylic acid esters of vinyl alcohol. Thus, according to DE 11 47 799 B1, oil-soluble copolymers of ethylene and vinyl acetate, having molecular masses between about 1,000 and about 3,000 g/mol, are added to petroleum distillate propellants or fuels having a boiling range between about 120.degree. C. and 400.degree. C. Copolymers which contain about 60% to 99% by weight of ethylene and about 1% to 40% by weight of vinyl acetate are preferred. They are particularly effective if they have been prepared by free radical polymerization in an inert solvent at temperatures of about 70.degree. C. to about 130.degree. C. under pressures of 35 to 2,100 atmospheres gauge (DE 19 14 756 B2).
Other polymers employed as agents which improve flow contain, for example, 1-hexene (cf. EP 184,083 B1) or diisobutylene (cf. EP 203,554 B1), in addition to ethylene and vinyl acetate. Copolymers of ethylene, alkenecarboxylic acid esters, vinyl esters and/or vinyl ketones are also used as pour-point depressants and for improving the flow properties of crude oils and middle distillates thereof (cf. EP 111,883 B1).
The effectiveness of the known additives for improving the properties of mineral oil fractions depends inter alia, on the origin of the mineral oil from which they have been obtained and, in particular, on the composition thereof. Additives which are outstandingly suitable for establishing certain values of the properties of fractions of one crude oil can therefore lead to completely unsatisfactory results on distillates of crude oils of different origins.
Additives which have a wide range of application, i.e. which effectively suppress precipitation of paraffins from mineral oils and mineral oil fractions of differing origin, have since become available. Nevertheless, there are cases in which they prove to be of little or even no value, either because they contribute little toward increasing the flow properties at low temperatures, they impair the filterability of mineral oil distillates above the cloud point, and/or they can be handled only unsatisfactorily. The reasons for this are diverse; the opening up of new raw materials and the changed processing of primary products may be mentioned as examples.
There is, therefore, a need for novel additives for improving the flow properties of those petroleums or petroleum fractions on which the additives of the prior art have little or even no effect. The additives furthermore guarantee adequate filterability of petroleum distillates above the cloud point and be usable without problems.